During the past 10 years, the applicant has isolated and partially characterized many large, polyhedral, dsDNA (>300kb) containing, plaque forming viruses which infect certain unicellular, eucaryotic, chlorella-like green algae. These viruses have several unique features, two of which are the focus of this proposal. (i) They encode DNA methyltransferases and DNA site- specific (restriction) endonucleases. Some of these endonucleases recognize and cleave DNA at the same position as bacterial restriction endonucleases, whereas others have novel specificities. The viruses also contain nonfunctional DNA methyltransferase genes; the applicant suspects they contain nonfunctional restriction endonuclease genes as well. (ii) The viruses contain at least three glycoproteins, including he major capsid protein.However, unlike other glycoprotein containing viruses, the chlorella viruses appear to encode putative glycosyltransferases involved in their glycosylation. The objectives of this proposal are: (i) To continue to isolate and characterize virus encoded DNA methyltransferases and DNA restriction endonucleases suspected of recognizing unique DNA sequences and to clone and sequence their genes. (ii) To determine important domains such as the target recognition domain in selected DNA methyltransferases and DNA restriction endonucleases by comparing functional and nonfunctional genes and making fusion proteins and site-directed mutants. (iii) To determine if the high homology between certain chlorella virus and bacterial DNA methyltransferases indicates natural interkingdom gene exchange between bacteria and the chlorella viruses. (iv)To determine how the chlorella viruses maintain a constant G + C content despite having vastly different levels of 5-methylcytosine. (v) To determine by genetic, molecular genetic, immunological, and biochemical methods those genes and gene products encoded by the prototype virus, PBCV-1, which glycosylate the virus major capsid protein.